This invention relates to an air-conditioning system for aircraft for conditioning humidity-containing air under excess pressure for air-conditioning an aircraft cabin.
The fresh air for air-conditioning aircraft cabins is conditioned from the air tapped from the engine at high pressure and high temperature, the so-called tap air. The air-conditioning systems utilize the pressure and temperature potential of the engine air for generating the required cooling capacity. The tap air is cooled in the course of the process of conditioning fresh air, is dehumidified and expanded to the cabin pressure of about 1 bar in ground operation and about 0.8 bar in flight operation. When conditioning fresh air, great importance is attached to air dehumidification, in order to prevent an icing of individual components of the air-conditioning system and in particular the formation of ice crystals in the fresh air to be conditioned. However, the necessity of dehumidification chiefly exists in ground operation, because in flight operation, i.e. in large altitudes, the ambient air and thus the tapped engine air is extremely dry in any case.
With reference to FIG. 1, an air-conditioning system is described below, as it is known for instance from DE 199 35 918 of the same applicant.
Via a flow control valve 12, that amount of tap air 10 of about 2 bar and 200xc2x0 C. is tapped from an engine, which is required for supplying fresh air to the cabin. In ground operation, the tap air is withdrawn from an auxiliary engine with about 3 bar. The tap air is first of all passed over a primary heat exchanger 14 and cooled to about 80xc2x0 C. The heat exchanger is an airxe2x80x94air heat exchanger, and as cooling medium ambient air 16 is used. In ground operation, the volume flow of the cooling air 16 is adjusted via a fan 18. In flight operation, the ram air supplied is sufficient as cooling medium, the volume flow being adjustable via a throttle valve. The tap air cooled to about 80xc2x0 C. is compressed in a first compressor C1 and proceeding from the same is further compressed in a second compressor C2 to about 5 bar. In a main heat exchanger 20, likewise an airxe2x80x94air heat exchanger, this pressurized air coming from the second compressor C2 is cooled to about 50xc2x0 C. by means of ambient air 16. The high pressure of about 5 bar is required for realizing a high degree of dehumidification in the subsequent water separation cycle. Therefore, this so-called aircycle system is also known as xe2x80x9chigh-pressure water separation cyclexe2x80x9d.
The high-pressure water separation cycle comprises a condenser 22, as it is also proposed for instance in EP 0,019,492 A, and a water separator 24 succeeding the condenser 22. The compressed, cooled tap air is cooled in the condenser 22 by about 15 K, water being condensed at the same time. The condensed water is then separated in the water separator 24. The air thus dehumidified is passed over two expansion turbines T1 and T2, the air being expanded to a cabin pressure of about 1 bar. Yet before the tap air emerging from the first turbine is supplied to the second expansion turbine, it is passed in a heat-exchanging manner through the condenser 22 of the high-pressure water separation cycle, in order to cool the compressed, cooled tap air to the temperature necessary for separating water in the water separator 24. The air expanded and cooled in the expansion turbine T1 is heated at the same time corresponding to the heat flow transferred in the condenser. In the high-pressure cycle, a heat exchanger 26 preceding the condenser 22 is provided in addition to the condenser 22. First of all, the compressed, cooled tap air is passed through the heat exchanger 26, before it enters the condenser 22, and subsequently the dehumidified air is passed through the heat exchanger 26, before it enters the expansion turbine T1. The main function of the heat exchanger 26 is to heat the dehumidified air and evaporate residual humidity while recovering energy at the same time, before the air enters the turbine T1. At the same time, however, the condenser 22 is relieved by the heat exchanger 26 in that before entering the condenser 22 the compressed tap air is additionally precooled by about 5 K.
The conditioned air emerging from the second turbine T2 at about xe2x88x9210xc2x0 C. and about ambient pressure is then mixed with recirculated cabin air in a mixing chamber which is not represented.
What is typical for an air-conditioning system as described herein is the fact that the energy recovered in the expansion turbines T1 and T2 is utilized for driving on the one hand the compressor C2 and C1, respectively, and on the other hand in addition the fan 18. In one case, three wheels, i.e. turbine (T2)/compressor (C1)/fan are arranged on a common shaft and form what is called the aircycle machine ACM, which is also referred to as three-wheel machine. The expansion turbine T1 together with the compressor C2 is arranged on a common shaft, but separate from the aforementioned three wheels. Therefore, this combined machine as a whole is also entirely referred to as 2+3-wheel machine.
The total system is designed for ground operation at an ambient temperature of, for instance, 38xc2x0 C. To optimize the effectiveness of the heat exchanging process in the cooling shaft 17, the water obtained in the high-pressure water separation cycle with a temperature of about t=30xc2x0 C. and a pressure of about 5 bar in the cooling shaft is supplied in fine droplets to be evaporated in said cooling shaft, whereby the effectiveness of the heat exchangers 20 and 14 is improved.
By means of a bypass means 28, the highly pressurized air originating from the main heat exchanger 20 can directly be supplied to the second expansion turbine T2, without passing through the water separation cycle. This may be of interest in particular when the tapped air is so dry already that it need no longer be dehumidified. This is the case in particular when flying in large flight altitudes.
With the water separation cycle from the known air-conditioning system described above, a sufficiently dry air can be achieved. It is, however, disadvantageous that the condenser and the preceding heat exchanger for dehumidifying the highly pressurized humid air have a large size. This is true in particular for the heat exchanger, as here only a small temperature gradient xcex94T is available for the heat transfer function. In aircraft technology, however, it is the foremost premise to build as small and lightweight as possible.
It is therefor the object of the present invention to develop an air-conditioning system for aircraft in accordance with
at least one compressor (C1, C2) for compressing the air (10) already supplied under an excess pressure to an even higher pressure,
a first expansion turbine (T1) for expanding the air to a lower pressure,
and a second expansion turbine (T2) succeeding said first one for the further expansion of the air,
such that the dimension of the entire device is reduced and the weight thereof can be reduced on the whole.
In accordance with the invention, this object is solved proceeding from an air conditioning system above by the combination with the features between the first expansion turbine (T1) and the second expansion turbine (T2) a droplet coalescing device (30) with succeeding water separator (32) is disposed.
Conceptually, the solution of the above object is achieved in that the construction of the water separation cycle is changed. Instead of the large-size condenser with preceding heat exchanger a droplet coalescing device is used, behind which a water separator is provided in a manner known per se. This constructional unit used for dehydration is incorporated after the first expansion turbine. In accordance with the invention, the highly pressurized, but still humid air coming from the main heat exchanger is supplied to the first expansion turbine at about 45xc2x0 C. The still humid air, which has been expanded in a first stage and in which the humidity has condensed in very fine droplets due to the decrease in temperature, is introduced from the expansion turbine into a droplet coalescing device which is used for coalescing the microfine mist-like droplets to form larger droplets, and these large droplets can subsequently be separated in the water separator. In the most simple case, the droplet coalescing device can consist of an elbow or of simple baffle plates. The turbulent air flow emerging from the expansion turbine T1, in which air flow the finely divided droplets are contained, is directed against said baffle plates. Due to the spin of the air flow or the air vortexes, the water droplets are flung onto the tube wall and the baffle plates and combined to form larger droplets on the surface thereof. These droplets are entrained by the flow towards the water separator and separated at the same. By means of the device a dehumidification of the air of 90 to 95% can be effected.
Behind the droplet coalescing device with succeeding water separator a heat exchanger may be provided, in which the air flowing from the first to the second compressor heats the air originating from the first expansion turbine and meanwhile dehumidified in the water separator, before said air is introduced into the second expansion turbine. By means of this measure, the water possibly still contained in the air is transferred to the gas phase. Thus, the air flowing into the second expansion turbine definitely no longer contains any free water. Possibly existing free water can lead to the erosion of the turbine nozzles or, for the case of outlet temperatures below the freezing point, to the icing of the second expansion turbine. At the same time, the air is heated before entering the second expansion turbine, whereby an increased turbine output is achieved. As an additional side effect, it is also achieved due to this cycle that the air originating from the first compressor is subjected to an intermediate cooling, whereby the efficiency and in particular the degree of water separation of the air-conditioning system is furthermore improved.
To improve the exchange efficiency of the heat exchanger, the same may be divided in two parts, where in the second part of the heat exchanger the air guided between the first and the second compressor preheats the air entering the first expansion turbine. The air is thereby brought to a higher temperature level, whereby the turbine output is improved.
In accordance with another embodiment of the invention, a second water separator may be provided, by means of which the air introduced into the first expansion turbine is at least partly dehydrated, before it is introduced into the heat exchanger, which like the remaining heat exchangers constitutes a regenerative heat exchanger. For the case that free water is already expected in the highly pressurized air, said free water can be withdrawn, so that the further highly pressurized water separation cycle is not additionally loaded by this free water.
In the air-conditioning system, a first bypass means may be provided for bypassing the first expansion turbine, in which case the air can directly be supplied to the second expansion turbine. The water separation cycle is bypassed in this way. This is possible in particular when the tap air consists of dry ambient air, for instance of ambient air in a large flight altitude.
For the case that the two-wheel system comprising the first expansion turbine and the second compressor should fail, the highly pressurized air can be passed via a second bypass device not into the first turbine, but directly into the droplet coalescing device, which according to this embodiment comprises an airxe2x80x94air heat exchanger. The highly pressurized, comparatively warm air is cooled by the cold air likewise passed through the heat exchanger, which cold air is supplied from the second expansion turbine and is likewise passed through the heat exchanger. As a result, free water is condensed in the highly pressurized air, which free water can largely be separated in the succeeding water separator.
For the case that the residual humidity content of the air introduced into the aircraft cabin is not subject to very high demands, the air emerging from the first expansion turbine can, in accordance with an alternative embodiment of the invention, directly be introduced into the water separator disposed between the first expansion turbine and the second expansion turbine. In this application, the coalescing device can be omitted. Surprisingly, experiments have shown that the turbulent flow emerging from the turbine likewise tends to coalesce to form larger droplets in the subsequent tube or elbow and at the walls as well as the spin means of the water separator, so that part of the free water in the water separator can also be withdrawn if no separately provided coalescing device is preceding said water separator.
Furthermore, protection is claimed for a device for coalescing microfine droplets, in particular for use in an air-conditioning system for aircraft with the aforementioned features. The structure of this coalescing device includes an airxe2x80x94air heat exchanger. Tests have confirmed that very fine mist-like droplets contained in the free water at the walls of the heat exchanger are coalesced and entrained by the flow. The larger coalesced droplets can then be separated in a water separator. In an airxe2x80x94air heat exchanger, approximately wave-shaped sheets are usually disposed between the parallel walls to increase the heat exchange surface. In particular at these sheets protruding into the flow, the fine droplets coalesce to form larger droplets.
In accordance with a further preferred aspect of the invention, a heat exchanger packing used as coalescing device is disposed in a housing, where a passage disposed in parallel to the heat exchanger packing is spared. Via a pivotally mounted flap, the air supplied can wholly or partly be passed over the heat exchanger packing or guided past the same in the free passage inside the housing.